One form of ultrasonic welding apparatus includes an ultrasonic sonotrode for pressing flat workpieces to be welded, against a backing or counter plate. Such an ultrasonic welding apparatus for a strapping machine is to be found for example in DE 39 11 634 A1, DE 39 34 560 A1, DE 196 14 634 A1 and EP 0 551 244 A1 to which reference is hereby directed.
In the case of those ultrasonic welding apparatuses, the surface of the ultrasonic sonotrode has projections thereon. In the EP 0 551 244 A1 the surface of the backing plate is also provided with projections. The projections may be for example in the form of ribs of a generally triangular cross-sectional configuration. At the beginning of each ultrasonic welding operation, each rib produces a high level of local contact pressure. That ensures that, in the region of the respective projection on the sonotrode, a high proportion of the ultrasonic energy is introduced into the respective workpiece to be welded and the material of the workpiece to be welded, usually a tape comprising a thermoplastic material, is subjected to initial melting in that region. Starting from that melting region at the beginning of the welding operation in the proximity of the respective ribs, the size of the region which has been melted under the effect of the ultrasound, with increasing input of energy into the welding location, progressively increases until the desired effect of welding through the entire welding region has been achieved. After the welding procedure has been effected the sonotrode is moved away from the backing plate and the tape loop is suitably released from the backing plate so that the welding apparatus is then available for welding together two new workpiece portions.
Welding apparatuses of that kind are used with great success at the present time as closure assemblies for strapping machines. They are however also suitable for other situations of use in which for example flat pieces of plastic material have to be joined together.
When welding strapping tapes, it is to be observed that the tensile strength of the welded join is basically below the tensile strength of the plastic tape itself. In addition a high degree of variation in the tensile strengths of the welded joins is observed. It is desirable to provide a welding apparatus for welding by means of ultrasound, which ensures more reliable welding. It is also desirable to provide an ultrasonic welding apparatus which is effective to provide for progressive formation of the welded join between two workpiece portions and to provide an ultrasonic welding apparatus which is operable to produce welded joins of greater consistency. It is also desirable to provide a closure assembly for a strapping machine for welding a loop of a strapping tape, including an ultrasonic welding apparatus according to the invention.